Process for the production of active lead dioxide

ABSTRACT

Suspending lead sulphate, lead carbonate or mixtures thereof in water and adding to said suspension a halide and alkali in such quantitative proportions to obtain a higher basic lead halide or lead halide carbonate and thereafter oxidizing said compound with halogen or hypohalite under neutral or alkaline conditions thereby effecting precipitation of highly active lead dioxide.

United States Patent Beecken 1 Feb. 22, 1972 PROCESS FO THE O UC OFOREIGN PATENTS OR APPLICATIONS ACTIVE LEAD DIOXIDE 23,588 10/1882Germany ..23/l46 [72] Inventor: Hermann Beecken, Cologne-Stammheim,819,275 9/1959 Great Britain... Germany 73 Ass Farbeni brlk Ba Aktillsch ft gnee Leverkuzen gl g engese a Primary Examiner0scar R. VertizAssistant Examiner-Hoke S. Miller [22] Flledl Sep 1969 Attorney-Connollyand Hutz [2l1 Appl. No.: 856,354

[30] Foreign Application Priority Data [57] a 2 ABSTRACT 7 Oct 2 1963Germany 18 00 4392 Suspending lead sulphate, lead carbonate or mixturesthereof in water and adding to said suspension a halide and alkali in52] us. Cl ..23/146, 252/312 Such quantitative Proportions to obtain ahigher basic lead [51] Int. ..C0lg 21/08, BOlj 1/00 lide or lead halidecarbonate and thereafter oxidizing said [58] Field of Search 723/146;260/6, 13 R; 252/312 compound with halogen or hypohalite under neutralor kaline conditions thereby effecting precipitation of highly ac- [56]References Cited tive lead dioxide.

UNITED STATES PATENTS 1,506,633 8/1924 Grunbaum ..23/ 146 9 Claims, 1Drawing Figure 2,630,457 3/1953 Hansen et al. ..252/3l2 PAIENTEUFEB22I972 INVENTOR.

HERMANN BbECKEN BY A PROCESS FOR THE PRODUCTION OF ACTIVE LEAD DIOXIDEin the production of perchloric acid and perchlorates, A

product having special properties is frequently required for oxidationprocesses involving lead dioxide. In addition to a high degree ofpurity, the lead dioxide is also intended to show a high level ofactivity, i.e., extreme particle fineness. Thus, in the manufacture ofdyestuffs for example, not only the yield but also the purity of the endproduct is governed by the activity of the/lead dioxide'us'd. in somecases, oxidation with lead dioxide is only displaced in the rightdirection ifa product with the necessary reactivity is used. Forexample, sensitive quinones can only be obtained with a lead dioxidethat has been prepared by treating lead tetra-acetate with an excess ofwater (Chemische Beric hte 83, 1950, page 413).

Generally, lead dioxide can be obtained by oxidizing lead (ll)-salts, inwhich case oxidation may be carried out both electrochemically andchemically, for example in the presence of powerful oxidizing agentssuch as chlorine, bromine or hypochlorite. Lead dioxide is commerciallyproduced for example by oxidizing lead (ll)-acetate solutions withchloride of lime as the oxidizing agent. Another commercial process alsostarts with tetravalent lead. In this case, red lead is treated withdilute nitric acid. There are also some processes that start with lead(ID-chloride. In one known process for example, lead (lb-chloride isdissolved in a solution of calcium chloride and the solution thusobtained is poured into a solution of chlorinated lime with excesscalcium hydroxide and then boiled. ln other known processes, an aqueoussuspension of lead (ID-chloride and calcium hydroxide in a molar ratioof 1:1 is prepared, chlorine being introduced into the resultingmixture. ln a modification of these conventional processes, white leadoxychloride is initially prepared by trituration from lead (ll)-oxideand sodium chloride, and is subsequently boiled in a solution ofchlorinated lime.

1n order to improve the economics of oxidation-reactions involving leaddioxide, several other known processes are concerned with converting theresulting precipitated lead sulphate back into lead dioxide bychlorination either directly or indirectly by way of lead carbonate.According to US. Pat. No. 1,506,633, lead sulphate for example is heatedin concentrated sodium hydroxide and simultaneously treated withchlorine. British Pat. No. 819,275 discloses a process in which leadsulphate is initially converted into lead carbonate which issubsequently oxidized with chlorine in an alkaline medium.

Those conventional processes starting with lead sulphate and leadcarbonate have serious disadvantages attributable not only to theconsumption of large alkali and chlorine excesses but also to theformation of coarse, only moderately active products. Accordingly, theseproducts cannot be reused either for leucobase-oxidation, in particular,or for many other applications.

A process for the production of lead dioxide with a high level ofactivity by treating substantially insoluble lead salts in the presenceof alkali with halogens or hypohalites has now been found in which leadsulphate or lead carbonate, optionally following the removal ofimpurities, is reacted in aqueous suspension at temperatures of fromabout 0 to 60 C. with halides and alkalis in such quantitativeproportions that higher-basic lead halides are formed which have anatomic ratio of Pbzhalogen of 3.5:1 to 4:1 and generally correspond tothe approximate composition :PbX, 6PbO (X=Cl, Br or I), and in which upto 3 PhD molecules can be replaced by PbCO where lead carbonate is usedas the starting material. The resulting basic lead halides are thenoxidized with halogens or hypohalites in an alkaline medium attemperatures of from-about 0 C. to about 1 10 C. and the lead dioxidethus obtained is separated off from the reaction medium.

It is a particular advantage of the process of invention that it enableslead dioxides to be obtained even from substantially insoluble compoundsof bivalent lead in the same highly active form that hitherto could onlybe obtained by hydrolysing lead tetra-acetate. It is therefore possibledirectly to reuse as starting material the lead sulphate formed by theuse of lead dioxide for oxidation reactions without any need foradditional complicated process stages. In some cases only the impuritieshave to be removed.

Although the following description is given with particular reference tolead sulphate, it may readily be applied to lead carbonate, as shown inthe Examples.

The halide ions are usually supplied in the form of sodium chloride,although the choice is by no means restricted to this chloride; instead,any chlorides, bromides or even iodides, in-

cluding organic halides, which liberate the aforementioned halide ionsand do not promote secondary reactions under the reaction conditions,may also be used. LiCl, KCl, KBr, Kl. MgCl CaCl POCl C]; and acetylchloride are mentioned by way of illustration.

The hydroxyl ions are preferably added in the form of alkali liquors,sodium hydroxide in particular, although other suffciently stronglydissociated bases such as KOH and LiOH, for example, are also suitableprovided they do not give rise to the formation of troublesome secondaryproducts.

The oxidation is preferably carried out with a commercial sodiumhypochlorite solution, so-called bleaching liquor, which is alsoabundantly available as a waste product from chlorine absorption plantsof the kind used in industrial processes. Naturally, hypochloritesolutions of different origin may also be used. Thus, it is alsopossible to produce the hypochlorite solution directly in the reactionmedium. Hypobromite solutions may also be used. Table 1 demonstrates theeffect of the alkali chloride addition before the addition of the sodiumhypochlorite solution, which itself is known to have a high sodiumchloride content, upon the properties of the lead dioxide. 0.075 mol ofIbSO was reacted as described in Example 1 with a molar ratio of PbSO/NaOH/N aOCl of l:2.67:l.l.

Measured on the suspensions washed free from salts after standing for 14days in 4.5 cm. diameter ml. glass breakers.

' 'NaOH added first followed by NaCl after conversion of the PbSO, intoPbtOH l:

'"NaCl replaced by an equivalent quantity of 36% hydrochloric acid andcompensated by the addition ofmore alkali.

It has been found that the molar ratio of PbSO, to NaCl and the atomicratio of Pb to C1 can be varied within wide limits; in particular, therange of from 4:1 to 1:8 and beyond leads to finely dispersed leaddioxides. Narrow limits are imposed upon the molar ratio of PbSO toNaOl-l and the PbzOH ratio by the stoichiometry of the reactionequation:

PbSo,+NaOH+NaOCl- PbO +Na SO,+NaC1+H- O On the one hand, sodiumhydroxide must be present in such abundance as to guarantee theintermediate complete formation of the requisite higher-basic leadhalides with a Pb/halogen ratio of 3.5 to 4. This means that a pH valuein excess of l i has to be maintained. A pH-value not lower than 7,i.e., a neutral or alkaline pH, should also be maintained during theiroxidation with sodium hypochlorite or chlorine. On the other hand,however, the alkali excesses should not be too high in order to avoidthe formation of hexahydroxoplumbates-(IV) as end products. Accordingly,it is preferred to operate with a molar ratio of PbSO. to NaOH of fromabout 1:2.0 to 123.5.

The hexabasic lead halide is formed extremely quickly from lead sulphateat temperatures as low as around C. to 60 C. Although highertemperatures up to about the boiling point of the mixture may also beused, temperatures of this order are not economical. Even only 8 tominutes after the alkali has been added to an intensivelystirredsuspension of lead sulphate, for example, in dilute sodiumchloride solution at 20 C., the analyses of the solid product show anatomic PbzCl ratio of 3.5 to 4.0. The product does not undergo anychange in composition even after the mixture has been heated for hoursto 60 C. The same atomic ratio is also obtained in cases where bromideor iodide solutions, for example, of potassium bromide or potassiumiodide, are used, or in cases where lead carbonate is used as thestarting material. In this case, the solid product additionally containscarbonate. Oxidation of the hexabasic lead halide or basic lead halidecarbonate to the lead dioxide may be carried out at temperatures of upto 110 C., temperatures of from about to 60 C. being preferred. Attemperatures below 20 C., the reaction time is unnecessarily lengthened,whilst temperatures above 60 C. do not afford any advantages. Since thereaction times can be reduced to a few minutes at temperatures of from50 to 60 C., the process according to the invention may also readily becarried out continuously provided a suitable reactor is used.

The concentration of lead sulphate in the aqueous suspension shouldamount to between about 10 and 20 percent by weight.

In some cases lead sulphates or lead carbonates which have given to thisproblem. For example, it has been proposed to free the lead sulphateresidues from the organic material by roasting at temperatures around500C. and thereafter reducing them and grinding into a form suitable forreoxidation. Ac-

5 cording to another proposal, the resin-containing lead 10 plicatedwhen it is considered that, in extreme cases, the resin contents canexceed percent of the lead sulphate residue.

In one special embodiment of the process according to the invention,therefore, emulsifiers are added in quantities of up to about 8 percent,based on the PbSO to the aqueous suspension of lead sulphate from theproduction of triphenylmethane dyes for the purposes of purification. Ithas proved to be particularly suitable to use for this purposepolyglycol ethers of alkanols, alkanols and alkyl phenols and nonionicemulsifiers of the kind that contain a hydrophobic radical consisting ofone or more aromatic nuclei and optionally further substituted, such asthose described for example in US. Pat. No. 2,630,457 and in German Pat.No. l,l2l,8l4.

The advance embodied in the new regeneration process is 25 explainedwith reference to comparative data on the relative obtained by knownmethods.

The grain-size distribution curves are based on measurements with aSartorius sedimentation balance, in cases of extreme fineness using acentrifugal field.

The numbers in FIG. 1 refer to the Examples set out in Table 2. Thebracketed figures relate to the numbers of the Examples. The grain size(equivalent nodule diameter) is plotted in pro. on the abscissa axis,whilst the residue is plotted as a percentage on the ordinate axis,i.e., that component of the total amount of lead oxide obtained whoseparticle size exbeen precipitated from a complex reaction medium needceeds the particular abscissa value.

TABLE 2 [Relative optimal yields 01 CI. Acid Green 22 (Cl. No. 42170)from the oxidation 0! the ieuco compound with lead dioxide suspensionsoi different origin under standard conditions] Percent Relative PbO inopt optimal Number Pbo -suspension prepared from- PbO; prepared by thesolid- 6 yield 1 PbSOr anhydrous chemically pure Process acc. toinvention, Example 58 94. 3 1. 11 97- 4 2 Pbisolr-rasidue ifromProduction of 0.1. Process acc. to invention, Example 3a 91. 7 1.08 9 .8

as c reen 3 PbSOI-residue from production of 0.1. Processacc.toinvention,Exampie 3a 92.4 1.13 99.1

Acid Green 22. 4 Pliasoirrfislidue from production of CI. Process acc.to invention, Example 2 90.5 1. 35 99.5

as 0 no 5. 5 PbSO4-rcsidue from production of 0.1. Process acc. toinvention, Example 3a 93.2 1.13 99.1

Acid Green 8. 6 P1390; prepared from PbSO esidue of Process acc. toinvention,Example4a 91.2 1.09 95.6

0. 6. 7 Pllalsg -riesidue from production of Michler Process acc. toinvention, Example 3a 90. 2 1. 4 1000 y o 8 PbSO4-residue as No. 7 withNa-hypo- Comparison Example 3b 91.1 0.595 2 chlorite in dilute NaOH. 9PbSO by chlorination in NaOH U.S. Pat. 1,506,633 comparison Example 3c93.1 0.707 6 -0 0 PbCO; by chlorination in soda solution Brgish Pat.819,275 comparison Example 87.7 0. 725 11 PbCO by chlorination in diluteNaOH. Comparison Exam 1e 4b 91. 3 0.788 76- 8 2 Lead tetra-acetate byhydrolysis.. Kuilmmdz I. ammer comparison 95.8 1. 044 1- xarnp e Driedat in vacuo.

b e opt=Extinction value at maximum of the dye yield curve wheredilution is constant and the oxidation carried out with increasingquantities of lead dioxide under standardised conditions (ordinate: c atAm n.=664 m: abscissa-mol percent P110 based on the leuco compound).

* Based on No. 7. d Chem. Berlchte, vol. 83 (1950), pages 413414.

The process according to the invention is illustrated by the followingExamples:

EXAMPLE I 1.268 kg. of 100 percent sulphate (4.l8 mols), in the form ofthe moist sludge or pressed cake which is formed during the productionof C.l. Basic Blue 1(C.l. No. 42025) and which in addition to colorlesslittle dye has a dry weight consisting largely of resins that can onlybe dissolved in organic solvents, are stirred into 5.3 liters oftapwater, followed by the addition of 0.20 kg. of 78 percent sulphuricacid and 0.04 kg. of an emulsifier prepared by ethoxylating3-benzyl-4-hydroxydiphenyl (US. Pat. No. 2,630,457), and then by 2 hoursintensive stirring at 60 to 70 C. The product is then suction-filteredwhile still hot and washed thoroughly with hot water until an almostcolourless filtrate runs off. A pale blueish PbSO,-sludge (21.9 percentof water; 99.25 percent of PbSO in the dry substance as against 84.03percent before purifica tion) is obtained in a yield in excess of 99percent. 87.8 g. of common salt (1.50 mol) are then added with intensivestirring (peripheral speed of stirrer Z 100 cm./sec., its walls and basebeing thoroughly covered) to a homogeneous suspension of 291.5 g. of thepurified lead sulphate sludge (0.745 mol of Pb- SO,) in 1 liter of tapwater, and a mixture of 150 g. of 50 percent sodium hydroxide (1.875mol) and 500 ml. of tapwater is run in quickly after a few minutes. Afew minutes later, a pure white paste of basic lead chloride begins toform with some initial signs of flocculation. After the alkali has beenadded, the

paste is stirred for another 1 to 2 hours, 584 g. of a 10.1 percentsodium hypochlorite solution are added (1 percent of the theoretical).and the mixture is heated to 30-35 C., shortening the oxidation time tobetween 2 and 3 hours. During this period, the main reaction takes place(after about 1 hour), being accompanied by a slight increase intemperature of 5 to 10 C., whilst the suspension passes through anolive, dirtybrown appearance. About 30 minutes after the main reactionhas begun, there are clear signs of the particles brightening anddecomposing into a light, extremely fine PbO suspension.

The presence of excess hypochlorite and, provided the excess isconstant, the end of the reaction too are ensured by iodometrictitration of samples of the overlay clarified by centrifuging and takenat 30-minute intervals. The lead dioxide suspension is obtained neutraland free from chlorine and chloride with four to five washes, restirringwith water in each case to twice the reaction volume (this amounts toabout 2.3 to 2.5 liters), allowing to settle and running offthe overlay.Yield: 802.6 g. of 21.42 percent lead dioxide suspension, correspondingto 174.9 g. of 100 percent PbO, (98.1 percent of the theoretical).

The lead dioxide content of the solid and the relative maximum yield inthe production of C.l. Acid Green 22 (Cl. No.

42170) are shown in Table 2 and the percentage grainsize distributionofthe suspension in FIG. 1.

EXAMPLE 2 Following the procedure of Example 1, 4.18 mols of leadsulphate are again used, except that this occasion the lead sulphateoriginates from the production of Cl. Basic Blue 5 (CI. No. 42140) andhas an extremely high resin content in its dry state. lnstead of theemulsifier used in the previous Example, 0.10 kg. of the emulsifierprepared in accordance with Example 9 of German Pat. No. 1,121,814 isadded. After washing, a pale blue lead sulphate sludge, 97.75 percent ofwhose dry weight consists of PbSG, as against 73.21 percent beforepurification, is obtained in an almost quantitative yield.

The contaminated sludge is dark grey-green in color and in spite ofevery attempt at different methods of purification, for example, byheating with dilute acids in the absence of an emulsifier, melts at 50to 60 C. to form a tarlike mass. Neither is it possible to liberate thelead sulphate grain from its resinlike wrapping at low temperatures,with the result that any attempt at reoxidation fails.

792 g. of lead sulphate sludge thus purified (corresponding to aninitial quantity of lead sulphate of 2.20 mols) are homogeneouslystirred into a solution of 525 g. of sodium chloride (9 mols) in 3liters of tapwater. after which a mixture of 450 g. of 50 percent sodiumhydroxide and 1.5 liters of water is run in and the mixture isthoroughly stirred for 2 hours at 25 to 30 C. 1.47 kg. of 11.7 percentcommercial sodium hypochlorite solution (1 10 percent of thetheoretical) are then added to the creamy paste which is then kept foranother 4 hours at 30 to 35 C. until the supernatant liquid does notshow any further appreciable reduction in its active chlorine content.After washing in the usual way. the yield comprises 2,545 g. of 20.61percent lead dioxide suspension corresponding to 524.5 g. of percent PbO(99.7 percent of the theoretical).

Generally, a fairly large number of lead sulphate residues. whose drysubstances have resin contents of up to approximately 3 percent, need nospecial prepurification before the oxidation process. Instead, it isoften sufficient to wash them free from dyes or to follow the procedureof Examples 1 and 2, dispensing with the addition of an emulsifier. Thelead sulphates sufficiently prepurified for oxidation by this kind ofacid treatment include, for example, the residues from the production ofMichlers hydrol, malachite green (C.l. Basic Green 4, Cl. No. 42,000),C.I. Acid Green 8 (Cl. No. 42,050) and Cl. Acid Green 22 (Cl. No.42,170).

EXAMPLE 3 a. 301 g. of lead sulphate sludge from the production ofMichlers hydrol (purified in accordance with Example 2 but in theabsence of an emulsifier; 75.6 percent dry substance with 99.1 percentof PbSO, corresponding to 0.746 mol of PbSO are stirred to form ahomogeneous suspension with 1 liter of tapwater to which g. of commonsalt (3 mols) have been added. To the suspension thus obtained is nowadded a mixture of g. of 45 percent sodium hydroxide and 500 ml. ofwater, followed by 2 hours intensive stirring. The product is oxidizedto lead dioxide by pouring in 500 g. of 13 percent sodium hypochloritesolution and heating to 30 C. A very light fine PbO is obtained which, 3hours after the sodium hypochlorite solution has been added, is washedwith water in the usual way.

Yield: 829.14 g. of 20.42 percent dioxide suspension corresponding to169.4 g. of 100 percent PbO (95.2 percent of the theoretical). PbOcontent of the solid (dried in vacuo at 60 C): 90.2 percent. Volume ofthe PbO -sediment after at least 24 hours settling: 650 ml. (as measuredin a 1 liter glass beaker).

b) The procedure described at (a) is repeated without the addition ofcommon salt, the PbSO being stirred with alkali for 24 hours rather than2 hours. The sediment volume remains practically unchanged although thelead sulphate is soon completely converted to lead hydroxide. Theproduct is then oxidized for 15 hours at 30 C. with 560 g. of 1 1percent sodium hypochlorite solution, dark brown coarse lead dioxidebeing obtained after washing.

Yield: 694.3 g. of 23.47 percent lead dioxide suspension correspondingto 163.0 g. of 100 percent PbO (91.7 percent of the theoretical) PbO-content of the solid: 91.1 percent. Volume of the PbO -sediment after24 hours: 170 ml.

c) Comparison Example according to US. Pat. No. 1,506,633:

301 g. of the same lead sulphate sludge as at (a) are introduced withstirring into a solution of 113 g. of sodium hydroxide (2.83 mols) in160 g. of water, so that a 30 percent sodium hydroxide solution isobtained allowing for the water present in the sulphate (73 g.).Chlorine gas is then introduced. the temperature of the reaction mediumbeing kept above 80 C. If the mixture becomes black-brown in color andif its pH-value falls to 6-7, the supply of chlorine is stopped and theproduct heated for another hour to 80-90 C., after which it is left tocool and the dark-colored coarse lead dioxide is washed in the usual wayby decantation.

Yield: 576.1 g. of 27.6 percent lead dioxide suspension corresponding to159.0 g. of 100 percent lbO (89.4 percent of the theoretical). Pbcontent of the solid: 93.1 percent. Volume of the PbO -sediment after 24hours: 163 ml.

The use of sodium hypochlorite solution instead of chlorine underotherwise the same conditions leads to a very similar result: dark-browncoarse-grained lead dioxide whose grainsize distribution curve is shownin FIG. 1 under N0. 9 1.

EXAMPLE 4 a) 276.8 kg. of deep-green lead sulphate sludge containingPb-dye compounds from the production of C.I.Acid Green 8 (C1. No.42,050), purified in accordance with Example 1 but in the absence of anemulsifier (82.2 percent dry substance with 96.85 percent of PbSO.corresponding to 0.727 kmol of PbSO are reacted with stirring to formlead carbonate by heating for 3 hours to approximately 80 C. in asolution of 100 kg. of anhydrous soda in 1,000 liters of tapwater. Thelead carbonate is then squeezed out and washed with water until analmost colorless filtrate is obtained. A lump-free suspension isprepared from the now pale green carbonate sludge by stirring it into asolution of 175 kg. of common salt in 1,000 liters of water. 150 kg. of50 percent sodium hydroxide which have been diluted beforehand with 500liters of water, are then added to the suspension with continuedvigorous stirring and 1.5 hours later 518 kg. of 11.3 percent commercialsodium hypochlorite solution are run into the resulting blueish, bulkyFound Pb 82.2 Found Pb/Cl=3.9l

Cl 3.6 Pb/CO2=2.644 CO1 7.2 i CO;/Cl= l .478 hr b) Comparison test withno chloride ions added: 1,558 g. (5.83 mols) of lead carbonate, in theform of the sludge obtained in accordance with 4a), are stirred withwater to form 7.5 liters of a suspension to which 1 liter of 50 percentsodium hydroxide (l9 mols) is slowly added at 20 to 25 C. The product isthen oxidized with thorough stirring at 20 C. to a maximum of 30 C. bythe introduction into it of chlorine at the rate of approximately 5liters per hour. After some 30 to 36 hours, the solution is still onlyweakly alkaline, so that another 320 ml. of 50 percent sodium hydroxide(6.08 mols) are added and more chlorine is introduced until the pH valueof the solution drops back to between 8 and 9. Oxidation is over when nomore bivalent lead is dissolved out after a suction-filtered sample ofthe lead dioxide has been heated with 10 percent acetic acid, anddetected following the addition of H SO After washing in the usual way,a dark brown, relatively coarse-grained lead dioxide suspension isobtained.

period of 1.5 hours at -90 C.

PbO content of the solid: 91.3 percent Volume of the PbO sediment of aquantity of the suspension corresponding to 0.75 mol of PbO (after 24hours): 180 ml.

c) Comparison Example according to British Pat. No. 819,275: 458 g. ofthe purified lead sulphate sludge also used in 3a) (corresponding to 343g. of PbSO =l. 13 mol) are initially heated with stirring for 3 hours to8090 C. in a solution of 265 g. ofsodium carbonate (2.50 mol) in 700 g.of water, after which 96 g. of chlorine (1.35 mol) are introduced over ainto the soda-alkaline suspension of the lead carbonate formed. Bychecking and adding a little soda where necessary, the pH value of themixture is prevented from falling below 7. The lead carbonate is soonconverted into brown-black lead dioxide which, by virtue of its particlecoarseness, can be washed by filtration under suction and treatment withwater on suction filter.

Yield: 636.1 g. of 33.5 percent lead dioxide suspension corresponding to213.0 g. of 100% l bO (78.9 percent of the theoretical). V7

PbO content of the solid: 87.7 percent Sediment volume of a quantity ofthe suspension corresponding to 0.75 mol of PbO (after 24 hours): 270m1.

a) 227.5 g. (0.75 mol) of anhydrous lead sulphate (chemically pure,precipitated) are introduced into a thoroughly stirred solution of 87.8g. of sodium chloride (1.5 mol: chemically pure) in 1 liter of distilledwater (3-liter glass beaker with an anchor stirrer effectively coveringthe walls). After a lumpfree suspension has been obtained, a mixture of180 g. of 45 percent sodium hydroxide (1.875 mols) and 500 ml. ofdistilled water is added. This is followed by stirring for 1 hour at 20C., after which a sample (A) is taken. After another 2 hours heating to60 C., a second sample (B) is taken. The two samples are carefullywashed free from salts and alkali, dried in vacuo at 40 C. and thenanalyzed. There are no signs of any sulphate ions in either. PbCl -61617.2)

Calculated: Pb 89.63%, Cl 4.38%, atomic Pb2Cl ratio 3.50 Found Pb87.30%, Cl 4.20%, atomic PbzCl ratio 3.57 (A) Found Pb 87.57%,C1 4.30%,atomic PbzCl ratio 3.56 (B) For oxidation, 500 g. of an 1 1.79 percentcommercial sodium hypochlorite solution (corresponding to 58.5 g. ofchlorine=l10 percent of the theoretical) are added to the pastysuspension which is then stirred for 1 hour at 60 C. Even after only 15minutes, only a minimal reduction in the active chlorine content canstill be iodometrically detected in the supernatant liquid centrifugedoff, the titre remaining constant after 25 minutes. A very light,yellowish-brown fine PbO suspension is obtained which, after four tofive washes with water to a washing volume of 5 liters (reaction volumeapproximately 2.3 to 2.5 liters), is neutral and free from chlorine(ide).

Yield 842.0 g. of 20.0 percent lead dioxide suspension corresponding to168.4 g. of 100% PbO- (94.6 percent of the theoretical).

lnstead of lead sulphate, the carbonate may be used without any changein the result, or alternatively these compounds may be precipitatedbeforehand from soluble lead compounds and the following procedure forexample adopted:

b) A solution of 100 g. of sodium sulphate in 500 ml. of water is addedto 750 ml. of 1m Pb(NO -solution. g. of

sodium chloride and a solution of 75 g. of sodium hydroxide in 350 ml.of water are then added to the resulting Pbso -suspension with vigorousstirring in the usual way. After 2 hours, the product is oxidized for 3hours at 30 C. with 500 g. of an 1 1.7 percent sodium hypochloritesolution. After the very fine light brown suspension has been washed,the yield comprises 876.6 g. of 20.0 percent lead dioxide suspension,corresponding to 175.2 g. of 100% PbO (97.7 percent of the theoretical).PbO content of the solid: 93.6 percent Volume of the PbO -sediment(after 24 hours): 595 ml.

EXAMPLE 6 272.6 g. of prepurified lead sulphate paste (83.46 percent drysubstance corresponding to 0.75 mol of PbSO are homogeneously stirredwith 1 liter of water, 50.6 g. (0.375 mol.) of sulphuryl chloride arepoured in, followed by the addition of a mixture of 300 g. of 45 percentsodium hydroxide (3.375 mol) and 500 m1. of water. The product is thenintensively stirred for 1 hour at 30 C., followed by the addition of 585g. of 10.0 percent sodium hypochlorite solution (1 percent of thetheoretical) for the purposes of oxidation. After stirring for 3 hoursat 30 C., a very fine and light lead dioxide suspension is obtained,being washed in the usual way.

Yield: 682.1 g. of 25.22 percent lead dioxide suspension correspondingto 172.0 g. of 100% PbO (95.7 percent of the theoretical).

Pbo -content of the solid: 93.4 percent Volume of the PbO -sediment(after 24 hours): 450 ml.

EXAMPLE 7 272.6 g. of lead sulphate sludge (0.75 mol of PbSO cf. Example6) are stirred into 1 liter of water, followed by the addition first of9.54 g. (0.225 mol)of1ithium chloride and then of 146.7 g. of 45 percentsodium hydroxide (1.65 mol), diluted with 500 ml. of water. The productis then stirred for 30 minutes at 30 C. After 4.5 hours oxidation at 30C. as in Example 6 and the usual wash, a finely dispersed, light PbOsuspension is obtained.

Yield: 794.5 g. of 21.8 percent lead dioxide suspension corresponding to173.3 g. of 100% PbO (96.6 percent of the theoretical).

Pbo -content of the solid: 93.0 percent Volume of the PbO -Sediment(after 24 hours): 515 ml.

EXAMPLE 8 44.6 g. of potassium bromide (0.375 mol) and 1 liter ofdistilled water are added to 272.6 g. of lead sulphate paste (0.75 molof PbSO cf. Example 6) and the mixture is stirred to form a homogeneoussuspension and thereafter a mixture of 180 g. of 45 percent sodiumhydroxide (1.875 mol) and 500 ml. of water is added. The product isstirred for 2 hours at to C. and then oxidized for 4 hours at C. with acommercial sodium hypochlorite solution as described in Example 7. Afterthe very light and fine suspension has been washed, the yield comprises:

696.1 g. ofa 24.76% PbO suspension corresponding to 172.2 g. of 100percent lead dioxide (96.0 percent of the theoretical).

PbO -content of the solid: 97.7 percent.

Volume of the PbO -Sediment (after 24 hours): 450 ml. Analysis of thebasic lead bromide: Approximately 40 m1. of the suspension of the basicbromide are removed just before the sodium hypochlorite solution isadded, suction-filtered and washed free from salts and alkali. Thefinely crystalline filter residue tinged with pale yellow was dried invacuo at 40 C. for analysis (4.0 g).

PbBrg'fi PbO (1706.15)

Calculated: Pb 85.0%, Br 9.37%, atomic Pb:Br ratio 3.50 Found: Pb 83.1%,Br 9.40%, atomic Pb:Br ratio =3,4l

EXAMPLE 9 272.6 g. of lead sulphate paste (0.75 mol of PbSO cf. Example6) are stirred with 1 liter ofwater, followed by the addition first of35.7 g. of potassium iodide (0.215 mol) and then 10 minutes later ofamixture of 147.0 g. of 45 percent sodium hydroxide (1.653 mol) and 500m1. of water. After 1 to 2 hours intensive stirring, sodium hypochloritesolution is added as usual to the pale yellow bulky basic iodide formed.In contrast to the previous Examples, 1 10 percent of the theoreticalquantity of sodium hypochlorite solution does not promote anyappreciable oxidation of the lead (ID-compound, even at C., due to theformation of hypoiodite, although oxidation does proceed smoothly at 30to 35 C. when a relatively large excess of sodium hypochlorite solution(in this case 993.0 g. of 1 1.8 percent commercial sodium hypochloritesolution, corresponding to 1 17.0 g. of chlorine or 220 percent of thetheoretical) is used. After washing in the usual way, a finely dispersedlight brown PbO is obtained.

Yield: 1025.0 g. of 16.41 percent lead dioxide suspension correspondingto 168.2 g. of PbO (93.8 percent of the theoretical).

PbO -content of the solid: 91.6 percent Volume of the PbO -sediment(after 24 hours) 524 ml. Analysis of the hexabasic lead iodide (preparedin accordance with Example 9, but with a molar ratio of PbSO:Kl:NaOl-l=l: 0. 5:3.375):

Pbl '6 PbO (1,800.14)

Calculated: Pb 80.5%,1 14.1%, atomic Pb:l ratio 3.50 Found: Pb 79.2%,]14.1%, atomic Pbzl ratio 3.44

The addition of sodium fluoride instead of the bromide in Example 8 doesnot yield a basic salt and, accordingly, on completion of oxidation,gives a dark brown, relatively coarse and substantially inactive leaddioxide.

EXAMPLE 10 Comparison Example according to Chem. Ber. Vol. 83 (1950),pages 413-414:

133.0 g. of recrystallized lead tetra-acetate (0.30. mol) are carefullyrubbed in portions in a mortar with a total of 800 ml. of water and theportions combined in a glass beaker are made up with water to 2 litersand then intensively stirred for another 2 hours. The very fine, light,lead dioxide suspension is then washed with water in the usual way bydecantation. Yield: 621.1 g. of an 11.14 percent lead dioxidesuspension, corresponding to 69.2 g. of 100% PbO (96.4 percent of thetheoretical).

PbO content in the solid: 95.8 percent Volume of the PbO -sediment after24 hours: 200 ml. and 500 ml. based on 0.75 mol of PbO Grain-sizedistribution curve, cf. No. 12 in FIG. 1.

What is claimed is:

1. 1n the process of producing lead dioxide of high activity by treatingsubstantially insoluble lead salt selected from the group consisting oflead sulphate, lead carbonate or a mixture thereof in the presence ofalkali with a halogen or hypohalite, the improvement which comprisesforming an aqueous suspension of said lead salt and treating saidsuspension at a temperature of from 10 to 60 C. with a chloride in anatomic ratio of PbzCl of about 4:1 to 1:8 and with sodium hydroxide in amolar ratio of Pb saltzNaOH of from 1:2 to 1:3.5 to produce higher-basiclead halide, halide carbonate or a mixture thereof with an approximateatomic ratio of Pbzhalogen of between 3.4:1 and 40:1, thereafteroxidizing said higherbasic lead compound with halogen or hypohalite at atempera ture of from about 0 to C. and at a pH of more than 7 andrecovering resulting precipitated highly active lead dioxide.

2. The process of claim 1 wherein said oxidation is carried out withsodium hypochlorite solution.

3. The process of claim 2 wherein said oxidation is carried out at atemperature offrom 20 to 60 C.

polyglycol ether of an aliphatic alcohol or of a phenol.

7. The process of claim 4 wherein said emulsifier is an ethoxylatednonyl phenol.

8. The process of claim 4 wherein said emulsifier is an ethoxylatedbenzylhydroxydiphenyl.

9. The process of claim 4 wherein said emulsifier is an ethoxylateda-arylethyl phenol

2. The process of claim 1 wherein said oxidation is carried out withsodium hypochlorite solution.
 3. The process of claim 2 wherein saidoxidation is carried out at a temperature of from 20* to 60* C.
 4. Theprocess of claim 1 wherein inpurities in said substantially insolublelead salt are initially separated at a temperature of up to 100* C. inthe presence of an emulsifier in an amount of up to 10 percent byweight, based on the dry weight of the residue.
 5. The process of claim4 wherein said impurities are separated at a temperature of from about60* to 80* C.
 6. The process of claim 4 wherein said emulsifier is apolyglycol ether of an aliphatic alcohol or of a phenol.
 7. The processof claim 4 wherein said emulsifier is an ethoxylated nonyl phenol. 8.The process of claim 4 wherein said emulsifier is an ethoxylatedbenzylhydroxydiphenyl.
 9. The process of claim 4 wherein said emulsifieris an ethoxylated Alpha -arylethyl phenol.